1. Technical Field
The present invention relates in general to Code Division Multiple Access (CDMA) systems and in particular to a Dedicated Control Channel (DCCH) and a Supplemental Channel (SCH) in a CDMA system. Still more particularly, the present invention relates to a method and system for controlling power in a DCCH and a SCH of a CDMA system operating in Discontinuous Mode (DTX).
2. Description of the Related Art
The fast growth in the use of wireless means of communication has resulted in a complex process for world wide frequency allocations and a number of frequency allocation protocols. Code Division Multiple Access (CDMA) is one of the distinct digital channel sharing topologies which have emerged as a result of a growing need for more channels or more efficient use of channels in digital communication. In a typical CDMA system, a honeycomb type pattern of cells is created which utilizes the same range of radio frequencies. CDMA systems can utilize precisely the same frequency spectrum in all sectors. This allows a single frequency to serve multiple users. The CDMA air interface is very efficient in its use of the subscriber station transmitter power, enabling the widespread commercial use of low cost, lightweight, hand-held portable units that have vastly superior battery life. The technology is also very efficient in its link budgets, minimizing the number of base stations required for an excellent grade of service coverage. Also, CDMA""s use of soft handoff (occurring when a user passes across a cell boundary) nearly eliminates the annoyance of dropped calls, fading, and poor voice quality.
The specifications for CDMA operation are outline in the Electronic Industries Association/Telecommunications Industry Association (EIA/TIA) IS-95-A and TSB74 standards document entitled Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System or CDMA Principles of Spread Spectrum Communication, by Andrew J. Viterbi. Recent developments in CDMA technology have led to the development of the IS-2000 standard to replace the traditional IS-95 standard.
The cdma2000 family of standards specifies a spread-spectrum radio interface that uses CDMA technology to meet the requirements for third generation (3G) wireless communication systems. Development of the cdma2000 family of standards has, to the greatest extent possible, adhered to the current architecture by specifying different layers in different standards. The physical layer is specified in IS-2000-2, the Medium Access Control (MAC) layer in IS-2000-3, the Link Access Control (LAC) layer in IS-2000-4, and upper layer signaling in IS-2000-5. The standards in the family correspond to the CDMA layers, and include:
IS-2000-1 Introduction to cdma2000 standards for spread spectrum systems
IS-2000-2 Physical Layer Standard for cdma2000 spread spectrum systems
IS-2000-3 Medium Access Control (MAC) Standard for cdma2000 spread spectrum systems
IS-2000-4 Signaling Link Access Control (LAC) Standard for cdma2000 spread spectrum systems
IS-2000-5 Upper Layer (Layer 3) Signaling Standard for cdma2000 spread spectrum systems
In addition, the family includes a standard that specifies analog operation, to support dual-mode mobile stations and base stations:
IS-2000-6 Analog Signaling Standard for cdma2000 spread spectrum systems
During normal operation, CDMA systems utilize frame transfer by which data is stored and transferred in a frame protocol format. Each frame has an associated power control block of bits which assist in ensuring that the signals are transmitted with sufficient power to be recognized at the receiver end for a given Grade of Service (GOS). As a CDMA phone is transmitting, the receiving cell site is constantly measuring the signal strength of the transmission. When the signal weakens, the transceiver in the cell site transmits power control up commands to the phone, instructing it to increase its power. If the signal strengthens, power control down commands instruct the phone to decrease its power.
The IS-95 standard, defines procedures for a complex power control method, designed to save on battery life and help prevent co-channel interference. The IS-2000 standard has been introduced to replace the IS-95 standard and incorporates many of the power attributes of its predecessor. Throughout the remainder of this document, use of the term CDMA, cdma2000 or IS-2000 refers to the cdma2000 family of standards.
In IS-2000, power considerations are important for efficient use of the system. Thus, the IS-2000 standard includes a dedicated control channel (DCCH). Assuming an efficient power control method, the use of DCCH can provide up to 3.8 dB gain when no data is being sent. This gain is a result of discontinuous transmission (DTX). When there is no data to send, DCCH is able to stop transmitting everything but power control bits in frames, while the fundamental channel (FCH) has to transmit eighth rate frames. However, the DTX transmission on DCCH may often degrade the performance of outer loop power control (i.e., the mechanism which adjusts the target Eb/No on a per frame basis) on both the forward and reverse links. This decreases the potential gain achievable from the use of the DCCH and may result in a performance penalty.
Thus, two major problems exist in the use of DCCH in DTX mode. These are (1) problem in outer loop power control resulting in reduced capacity, and (2) possible call drop. Both problems stem from the receiver""s inability to distinguish between an erroneous frame and a cessation of transmission. In normal operating modes, when a frame error is detected at the Receiver Station, the receiver increases the target bit energy to noise spectrum density ratio (Eb/No). Otherwise, the target signal power level (Eb/No) is decreased. However, during DTX period, only power control bits are transmitted. In this case, the receiver is not able to distinguish between an error frame and a cessation of transmission. The misinterpretation of a transmission stoppage as a bad frame causes the receiver to increase its target Eb/No, and thereby commands the transmitter to power up unnecessarily. If the transmitter is smart enough, it can ignore this power up command as it knows it is in DTX mode. However, when the data transmission resumes, the target Eb/No at the receiver is too high. Consequently, the transmitter is forced to transmit data at a higher power level than necessary. In addition, the mobile unit stops transmission on the reverse link if it receives twelve consecutive erroneous frames on the forward link and it drops the call if there are no two consecutive good frames during any five second period. Therefore, DTX on the DCCH may also lead to possible call drops if no detection is used to determine when DTX is on or off.
Samsung has proposed two methods of frame error estimation of DCCH in DTX mode to try to solve the outer loop power control problem. These methods focus on the forward link and estimate the forward link error of the DCCH in DTX. The methods are summarized below.
1. Mobile System (MS) measures the average Eb/No using power control bits over a frame and maps it into frame error rate (FER) using a lookup table made from the additive white gaussian noise (AWGN) channel with a pre-determined offset value. Then the MS determines frame error in a stochastic way, i.e., generating a random number uniformly distributed over [0, 1] and comparing it with the FER. If the random number is larger than the FER, then the received frame is considered as a good frame. Otherwise, a frame error occurs.
2. MS measures the average Eb/No power control bits over one frame duration and compares the average Eb/No with the certain threshold value. If the average Eb/No is less than the threshold, the frame is considered bad. Otherwise the frame is considered good.
One obvious drawback with both methods is that while they may work well with a particular case for which they are optimized (eg., a particular MS speed and a channel condition), they will not work well with all other cases, such as, different MS speeds and propagation environments, since ithe relationship between Eb/No and frame error rate (FER) is not the same for different channel conditions.
Additionally, in the first method, a lookup table for Eb/No to FER mapping is very critical to the performance. In reality the Eb/No-FER relation varies widely with different propagation conditions. An inaccurate Eb/No-FER mapping could drastically degrade performance of the system using Samsung""s first approach.
In the second method, the value of the threshold is critical to the performance. It, in fact, also depends on the accuracy of Eb/No to FER mapping. If the value is set at the required Eb/No value for a given set of conditions, it will result in a very high average forward gain with no frame errors in some other conditions. Therefore, power is wasted. There is an optimal threshold at which the same performance and capacity could be achieved as usual forward link outer loop power control. However, this optimal value is highly sensitive to the mobile speed and propagation environment. There is no way to define a unified value. Moreover, a little bias in the threshold will lead to a big performance or capacity degradation. Evaluations of the performance of the second method yield results which are represented in the tables of Appendix B. Similar results can be expected with the first method.
The simulations of Appendix B indicate that the optimal threshold value in Samsung""s method is sensitive to the MS speed and location (interference environment). Although the Samsung methods solve the call drop issue, they degrade the system performance or capacity. Furthermore, they do not help on the forward link. It is also expected that the threshold varies with the number of multipaths and propagation environment. Therefore, there is no way to define a unified optimal threshold value in Samsung""s method. To meet the target FER in all scenarios, a worst case threshold could be defined in their method. However, this threshold value will sacrifice a lot of forward power in most cases.
The present invention thus recognizes that it would be desirable to have a method and system for preventing unnecessary power up of transmitters as a result of false readings of frame transmission during DTX. It would further be desirable to keep target Eb/No from unnecessarily increasing while also preventing the mobile unit from mistakenly stopping reverse transmission and/or dropping calls due to DTX.
It is therefore one object of the present invention to provide an improved CDMA system.
It is another object of the present invention to provide an improved power control method and system for a CDMA system.
It is yet another object of the present invention to provide an improved method and system for controlling power in a CDMA system operating in Discontinuous Mode (DTX).
The foregoing objects are achieved as is now described. A signalling method for controlling unnecessary power increases and call drop during discontinuous transmission (DTX) mode of a frame-based transmission system is provided. The signalling method comprises the steps of (1) detecting, at a receiver end of the transmission system a status of a transmitted frame indicating one of two possible transmission modes including (a) when a gating-off of the traffic channel occurs, and (b) when no gating-off of traffic occurs and normal traffic is being transmitted; and (2) controlling a change in the target bit energy to noise spectrum density ratio Eb/No in response to the detecting step so that a target Eb/No is increased only when the detecting step does not indicate a gating-off of traffic has occurred.
In a preferred embodiment of the invention, the detecting step includes calculating a ratio of a traffic signal-to-noise ratio (SNR) value to a SNR value of a power control bit stream over a frame period, establishing a threshold value correlated to a point above which the ratio indicates a normal frame is being transmitted, and comparing the ratio with the threshold. The target signal strength is then adjusted based on the results of the comparison.
In another preferred embodiment, the detecting step includes identifying one or more of a plurality of power control bits per frame as a DTX indicator and manipulating the bit to indicate to a receiver when a traffic channel in the frame is gated-off. When the frame arrives at the receiver the bit is analyzed and the target signal strength or bit energy to noise spectrum density (Eb/No) is adjusted accordingly.
The above as well as additional objects, features, and advantages of the present invention will become apparent in the following detailed written description.